1. Field of the Invention
The invention relates, generally, to a precision measuring system and, more specifically, to a spatial reference system for making dynamic and rapid measurements in one, two, or three dimensions simultaneously.
2. Description of the Related Art
Measurement techniques and strategies used to determine physical dimensions or other spatial quantities are well known. Some of the most common tools or devices that people associate with taking measurements include rulers, measuring tapes, marked vessels, scales, clocks, and stopwatches. These devices are relied upon to be accurate to a relative measurement standard. When greater measuring precision is required more accurate measuring devices must be employed, such as micrometers, vernier calipers, and electronic scales and balances, for example. These measuring devices fundamentally have greater precision, or a higher degree of accuracy, by their calibration to a finer or smaller degree of deviation from a known and stable standard. However, the accuracy of any precision measuring device also relates to its ability to maintain a stable and unchanging adherence to its initial calibration.
In the production and manufacturing industries, precision measurement is extremely important. Quantitative measurements relating to length, volume, and weight, also known as dimensional metrology, are fundamental to the manufacture of physical objects and devices as well as the control of a wide variety of processes. Production machines (i.e., milling machines, boring machines, high-speed assembly devices, etc), measurement related instruments used in production (i.e., alignment tools, micrometers, calipers, etc.), and movable production devices (i.e., robotic handling devices and welders, laser and water cutting devices, automated assemblers, etc.) all require precision calibration to provide the desired level of accuracy necessary to carry out their design function. For example, if it is desired to operate a repetitive precision milling process to produce a quantity of identical parts with a high degree of accuracy, the alignment of the various parts of the milling machine as well as the alignment of the cutting elements is critical to the process and must be maintained during the operation.
In a like manner, if it is desired to produce a physical object, or to repetitively reproduce an object from plans or an original, the object produced must meet the predetermined dimensions within an acceptable tolerance of precision from the original. This means that the produced objects themselves must be measured, either individually or by representative samplings to ensure that the accuracy of production is maintained throughout the process. The necessity for accuracy and precision in manufacturing processes highlights a number of drawbacks and disadvantages of conventionally available measuring devices.
In practice, the physical dimensions of a produced object or the alignment and calibration of a production machine may be measured through a variety of methods including the use of instruments, optical devices, and artifacts. When an artifact, also referred to as a test body, or specimen is used, a number of different approaches for precision measuring may be employed. Particularly, an artifact may be used as an exact, highly precise model, or copy, of a desired object in which the dimensions of the artifact are accurately followed to produce replicas. An artifact may also be the physical standard of a known, precise dimension to which other measuring devices are calibrated against, or the artifact may be used itself as the known dimensional standard to which other devices are aligned by, or calibrated against. Most often this latter type of artifact takes the form of a length bar or gauge block.
Length bars and long gauge blocks are among the most commonly used measuring standards for maintaining a repeatable precision in dimensional metrology. They are used to calibrate a wide range of instruments and other standards from micrometers to co-ordinate measuring machines (CMMs). Before use, all length bars and long gauge blocks must themselves be calibrated according to various specification standards such as governmental standards or engineering society standards. Length bars and long gauge blocks must also be re-checked and calibrated periodically to ensure that they maintain their accuracy. More importantly, they must also be used within a given temperature range or their calibration becomes uncertain due to thermal expansion. All materials, as an inherent physical property, have a unique coefficient of thermal expansion. In a linear manner, the coefficient represents the material""s expansion (or contraction) as a change in the length of the object per each unit length of the object for a one degree change in temperature. Thus, when the length bar or gauge block is used in differing temperature environments, the thermal expansion coefficient of the material of the artifact introduces a measurement uncertainty. In some circumstances, the changes due to thermal expansion of a measuring artifact may be constantly calculated and accounted for by the software of a particular production machine""s operating or controlling software. However, this requires additional sensors, monitoring devices, and compensation methods that are often not practical or are beyond the capability or capacity of most production devices. Thus, a calibrated length bar or gauge block has remained the typical measuring device.
The inherent drawback to using a calibrated length bar or gauge block is that the required calibration is a time consuming process that must be performed regularly if exacting standards for precision are to be maintained. Additionally, since the calibrated length bar or gauge block is only as good as the control of the temperature of the environment in which it is placed, these types of measuring artifacts must be kept at a controlled temperature. Otherwise, if the temperature of the ambient environment or the temperature of the artifact changes, its physical dimensions will change and the accuracy and precision of the artifact will be compromised. This often places stringent requirements over the control of the environment surrounding the manufacturing process. In certain manufacturing situations, the ambient environment is such that temperature control is difficult or unfeasible. In these circumstances several sets of artifacts are used, each artifact being maintained at a controlled temperature until its time of use. Thus, there is an ongoing need in the art to provide a measuring artifact that maintains its precision to the predetermined standard without deviation related to thermal influences and avoids the need for repetitive recalibration.
An additional consideration with the use of measuring artifacts, in relation to thermal coefficients of expansion, is the fact that any expansion of the artifact is a function of its overall length. In other words, uncertainty in the accuracy of the artifact is more difficult to control in long measurement artifacts as the coefficient of thermal expansion is relative to a unit length of the material in question. Thus, the longer the artifact, the greater its expansion for a particular change in temperature, making it difficult to measure longer alignment or calibrating distances with precision. This further highlights the need for an artifact that is constructed in such a manner that the coefficient of expansion is non-existent or is compensated out, not only over relatively short spans, but also longer measurement distances as well.
Another disadvantage with conventional measuring artifacts is that typical length bars and gauge blocks are good for a measurement standard along one-dimensional plane only (i.e., length, rather than length and width). However, many manufacturing situations require that an artifact provide precision locating or measurement not only along one axis but also along the second and/or third axes. In other words, it is desirable to have a measuring artifact or a combination of artifacts in a reference system that can provide precision locating in two and three-dimensional space (2D and 3D) at one time. Therefore, there exists a need to not only have a measuring artifact that overcomes the above-mentioned inadequacies but one that can be used in multiple combinations to provides a stable, highly precise spatial reference system, which functions in 2D and 3D spatial orientations, as well.
Certain attempts have been made at improving measuring systems along the lines of developing spatial (3D) measuring devices, as evidenced by German patents DE 11860883, DE 19708830, and DE 19915012. However, the body, or main portion, of the measuring artifacts in these designs are either constructed of certain types of carbon fiber based materials that are susceptible to humidity or glass based materials that are delicate and can easily be damaged. Moreover, if an artifact that is constructed of either of these types of material are inadvertently struck or bumped along their length, unseen internal stress cracks and fractures will result that alter, and increase, their thermal expansion properties.
These patents also disclose the use of other, relatively new, alloyed materials that are known to have much lower coefficients of thermal expansion than conventional metals, such as Invar, for example. However, these alloyed materials still exhibit some expansion and thereby leave room for improvement. Additionally, the end probes used in these designs utilize a number of component parts that have distinct thermal expansion coefficients that must be accounted for. While these designs do offer some improvements over length bars and gauge blocks, particularly by offering better spatial (2D and 3D) measuring capabilities, and improved dimensional stability regarding thermal influences, they still ultimately suffer the same shortcomings of uncertainty in measurement accuracy as all conventional measurement standards currently available. Therefore, beyond the simple use of materials at exhibit minimal thermal expansion, there still remains a need for an artifact that has a physical structure that completely compensates for all the expansion of its components such that there is no change in the length of the artifact.
The present invention overcomes the disadvantages of the related art by providing a spatial reference system that includes at least one artifact assembly. The artifact assembly has a measuring bar assembly including an inner member with a proximate end and a distal end, an outer member with a proximate end and a distal end, and a compensating member with a proximate end and a distal end operatively disposed between said inner and said outer members. The distal end of the outer member is fixedly mounted to the distal end of the compensating member. The proximate end of the compensating member is fixedly mounted to the proximate end of the inner member. The inner and the outer members each have a predetermined length and a predetermined coefficient of thermal expansion and the compensating member has a predetermined length and a predetermined coefficient of thermal expansion, such that the thermal expansion of the inner and the outer members is substantially eliminated by the expansion of the compensating member.
Thus, the present invention also overcomes the disadvantages of the related art by providing a measuring bar artifact that is thermal expansion compensated and can be combined with like artifacts to provide a spatial reference system for precision orienting, locating, and positioning in one, two, and three dimensions.